The invention relates generally to implementation of a transmission connection using digital modulation. The invention can be used in any system in which the bit stream to be transmitted is divided onto several carriers located on different frequency bands, but the invention is especially intended for implementation of a wireline transmission connection with the aid of VDSL (Very high bit rate Digital Subscriber Line) technology.
Optical fiber is a self-evident choice as transmission medium for a trunk network, because trunk connections usually need a high transmission capacity, the transmission distances used are long and existing routes are often found for the cables. Even for subscriber connections (the line between the local exchange and the subscriber) the situation is rapidly changing, because various services implemented with multimedia and demanding a high transmission rate will be everyday services also from the viewpoint of the private consumer.
However, no significant savings can be expected in the costs of constructing future networks offering broadband services, because the costs arise mainly from cable installation costs. However, it would be desirable to build as much optical fiber as possible also in subscriber networks, because it can be clearly seen that it will be needed in the future. The costs of renewing subscriber networks are very high, however, and in terms of time decades are in fact at issue in this context. High costs are indeed the worst obstacle to the spreading of the fiber into subscriber networks.
For the reasons mentioned above, more efficient measures than before have been taken in order to find out how to utilize the conventional subscriber line (the metal wire pair) for high speed data transmission, that is, for transmission rates clearly above the rate (144 kbit/s) of the ISDN basic access. The present ADSL (Asymmetrical Digital Subscriber Line) and HDSL (High bit rate Digital Subscriber Line) techniques do indeed offer new possibilities for high-rate data and video transmission along the wire pair of a telephone network to the subscribers"" terminals.
The ADSL transmission connection is asymmetrical in that the transmission rate from network to subscriber is considerably higher than from subscriber to network. This is due to the fact that the ADSL technique is intended mainly for various so-called xe2x80x9con-demandxe2x80x9d services. In practice, in the ADSL transmission connection the transmission rate from network to subscriber is in an order of 2 . . . 6 Mbit/s and from subscriber to network in an order of 16 . . . 640 kbit/s (a mere control channel).
The HDSL transmission technique concerns transmission in a metal wire pair of a digital signal of 2 Mbit/s level. HDSL represents a symmetrical technique, that is, the transmission rate is the same in both directions. The individual HDSL transceiver system comprises transceivers using echo-cancellation technology, which are interconnected by way of a bi-directional transmission path formed by a wire pair. In a HDSL transmission system the number of such individual transceiver systems may be one or two or three in parallel, whereby in a case of two or three pairs in parallel the rate to be used in each parallel transmission connection is 2 Mbit/s sub-rate; 784 kbit/s if there are three pairs in parallel and 1168 kbit/s if there are two pairs in parallel. It is defined in international recommendations how signals of 2 Mbit/s level are transmitted in a HDSL system, such as, for example, VC-12 signals of a SDH network or 2048 kbit/s signals in accordance with the G.703/G.704 recommendations of the CCITT.
Since only such bit rates are achieved with the solutions mentioned above, which are typically in the order of 1 . . . 2 Mbit/s, a technique allowing ATM level bit rates has been sought for the subscriber line cable. A specification of VDSL equipment is in fact being made by the international standards institute ETSI (European Telecommunications Standards Institute). The intention is that a VDSL transmission connection implemented in a metal wire pair of a telephone network will be able to transmit ATM cells between a telephone network and a subscriber terminal.
For example a VDSL connection can be implemented in such a way that predetermined frequency bands are allocated to both transmission directions and the data is modulated to the frequency bands of the transmission direction by generating a carrier for each frequency band in a separate modulator, in which the data is modulated in one phase to the final frequency band. The drawback of such a solution is that pulse shaping (interpolation and filtering) and frequency shifting (mixing) must be carried out in the modulator at a high frequency, which requires heavy computation and makes the equipment relatively complex. In the case of a VDSL connection, for example, the pulse shaping and mixing require computation operations to be performed at as high a rate as 30-fold sampling frequency, since some of the carriers are located at high frequencies. This will be described in detail hereinbelow.
The purpose of the present invention is to bring about an improvement on the drawback presented above by providing a solution allowing implementation of modulation and demodulation with as simple and efficient equipment as possible in systems of this type in which the bit stream to be transmitted is transported on carriers located on several different frequency bands.
This objective is achieved through a solution according to the invention which is defined in the independent claims.
The idea of the invention is to perform the frequency shift of signals to their final frequency bands by using a halfband filter bank that is computationally a very efficient design.
On account of the solution in accordance with the invention, the majority of signal processing required by digital modulation and demodulation can be carried out at low frequencies, since in transmission and reception the frequency shift is performed with a computationally efficient halfband filter bank.
In a method according to the invention some simple modulation method is utilized, preferably, for example, QAM (Quadrature Amplitude Modulation) or CAP (Carrierless Amplitude and Phase) modulation.